python - why can't x[:,0] = x[0] for a single row vector? -


i'm relatively new python i'm trying understand seems basic.

create vector:

x = np.linspace(0,2,3) out[38]: array([ 0.,  1.,  2.])

now why isn't x[:,0] value argument?

indexerror: invalid index

it must x[0]. have function calling calculates:

np.sqrt(x[:,0]**2 + x[:,1]**2 + x[:,2]**2)

why can't have true regardless of input? many other languages, independent of there being other rows in array. perhaps misunderstand fundamental - sorry if so. i'd avoid putting:

if len(x) == 1:     norm = np.sqrt(x[0]**2 + x[1]**2 + x[2]**2) else:     norm = np.sqrt(x[:,0]**2 + x[:,1]**2 + x[:,2]**2)

everywhere. surely there way around this... thanks.

edit: example of working in language matlab:

>> b = [1,2,3] b =      1     2     3  >> b(:,1) ans =      1 >> b(1) ans =       1

perhaps looking this:

np.sqrt(x[...,0]**2 + x[...,1]**2 + x[...,2]**2)

there can number of dimensions in place of ellipsis ...

see what python ellipsis object do?, , the docs of numpy basic slicing


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